The present invention relates to an Rxe2x80x94Txe2x80x94B sintered permanent magnet having high coercivity, residual magnetic flux density and maximum energy product.
With respect to Rxe2x80x94Txe2x80x94B sintered permanent magnets, wherein R is at least one rare earth element including Y, and T is Fe or Fe and Co, those having maximum energy products of about 40 MGOe are mass-produced. Means for adjusting the alloy compositions of the Rxe2x80x94Txe2x80x94B sintered permanent magnets include a single method and a blend method.
The single method is a method for producing an Rxe2x80x94Txe2x80x94B sintered permanent magnet using an ingot adjusted to have a main component composition of an Rxe2x80x94Txe2x80x94B sintered permanent magnet at a melting and/or casting stage, through the steps of pulverization, molding in a magnetic field, sintering and heat treatment. The resultant Rxe2x80x94Txe2x80x94B sintered permanent magnet is subjected to predetermined machining and surface treatment for use in practical applications.
The blend method is a method for producing an Rxe2x80x94Txe2x80x94B sintered permanent magnet through the steps of mixing of two or more types of Rxe2x80x94Txe2x80x94B sintered permanent magnet powder having different compositions at such a formulation as to provide the final Rxe2x80x94Txe2x80x94B sintered permanent magnet with a desired main component composition, pulverization, if necessary, and further molding in a magnetic field, sintering, heat treatment and surface treatment.
The above single method can relatively easily provide sintered permanent magnets with a high coercivity iHc, their residual magnetic flux density Br and maximum energy product (BH)max are low, unsuitable for applications requiring high Br and (BH)max.
Though conventionally proposed as applications of the blend method are an Rxe2x80x94Txe2x80x94B sintered permanent magnet produced from an Rxe2x80x94T alloy having a high R content and an Rxe2x80x94Txe2x80x94B alloy having a low R content (Japanese Patent Laid)pen No. 7-122413), and an Rxe2x80x94Txe2x80x94B sintered permanent magnet in which Ga, C and O are segregated in an R-rich phase and its vicinity (Japanese Patent Laid-Open No. 9-232121). However, there is still room for improvement to make them suitable for high-Br, high (BH)max applications. Particularly with respect to heavy rare earth elements having large influence on magnetic properties, their optimum concentration distributions in main phase particles and their control have not yet been made clear.
Accordingly, an object of the present invention is to provide a high-performance Rxe2x80x94Txe2x80x94B sintered permanent magnet suitable for applications requiring high Br and (BH)max.
Thus, the Rxe2x80x94Txe2x80x94B sintered permanent magnet according to the present invention has a composition comprising 28-33 weight % of R, and 0.5-2 weight % of B, the balance being substantially T and inevitable impurities, wherein R is at least one rare earth element including Y, at least one heavy rare earth element selected from the group consisting of Dy, Th and Ho being indispensable, and T is Fe or Fe and Co, the permanent magnet having a crystal structure comprising first R2T14B-type, main-phase crystal grain particles having a higher heavy rare earth element concentration than that of a crystal grain boundary phase, and second R2T14B-type, main-phase crystal grain particles having a lower heavy rare earth element concentration than that of the crystal grain boundary phase.
In a preferred embodiment of the present invention, the Rxe2x80x94Txe2x80x94B sintered permanent magnet has a composition comprising 28-33 weight % of R, 0.5-2 weight % of B, and 0.01-0.6 weight % of M1, wherein M1 is at least one element selected from the group consisting of Nb, Mo, W, V, Ta, Cr, Ti, Zr and Hf, the balance being substantially T and inevitable impurities.
In another preferred embodiment of the present invention, the Rxe2x80x94Txe2x80x94B sintered permanent magnet has a composition comprising 28-33 weight % of R, 0.5-2 weight % of B, 0.01-0.6 weight % of M1, and 0.01-0.3 weight % of M2, the balance being substantially T and inevitable impurities, wherein M, is at least one element selected from the group consisting of Nb, Mo, W, V, Ta, Cr, Ti, Zr and Hf, and M2 is at least one element selected from the group consisting of Al, Ga and Cu.
In a further preferred embodiment of the present invention, the Rxe2x80x94Txe2x80x94B sintered permanent magnet comprises more than 31% and 33% or less by weight of R, with 0.6 weight % or less of oxygen, 0.15 weight % or less of carbon, 0.03 weight % or less of nitrogen and 0.3 weight % or less of Ca as inevitable impurities.
In a still further preferred embodiment of the present invention, the Rxe2x80x94Txe2x80x94B sintered permanent magnet comprises 28-31 weight % of R with 0.25 weight % or less of oxygen, 0.15 weight % or less of carbon, 0.15 weight % or less of nitrogen and 0.3 weight % or less of Ca as inevitable impurities.
The Rxe2x80x94Txe2x80x94B sintered permanent magnet of the present invention is produced, for instance, by the steps of mixing of two types or more of alloy powder having substantially the same composition except for the difference in a ratio of heavy rare earth elements (Dy, etc.)/light rare earth elements (Nd, Pr, etc.) with the same total amount of the rare earth elements, molding in a magnetic field, sintering, heat treatment, and if necessary, machining, finish working such as barreling, etc., and surface treatment such as Ni plating, etc. Depending on the compositions of the above two types or more of alloy powder and the final composition of the Rxe2x80x94Txe2x80x94B sintered permanent magnet, the optimum sintering conditions are selected to strictly control the diffusion of heavy rare earth elements such as Dy in the crystal structure of the sintered magnet. As a result, the crystal structure has a characteristic concentration distribution of heavy rare earth elements such as Dy in the R2T14B-type, main-phase crystal grain particles (substantially in center portions) and the crystal grain boundary phase, containing R2T14B-type, main-phase crystal grain particles having a higher concentration of heavy rare earth elements such as Dy than that of the crystal grain boundary phase, and R2T14B-type, main-phase crystal grain particles having a lower concentration of heavy rare earth elements such as Dy than that of the crystal grain boundary phase.
The Rxe2x80x94Txe2x80x94B sintered permanent magnet having such a sintered crystal structure has extremely larger Br and (BH)max than those of the Rxe2x80x94Txe2x80x94B sintered permanent magnet produced by the single method, though its coercivity iHc is slightly smaller than that of the latter. Though the correlation between such high magnetic properties and the concentration distribution of heavy rare earth elements such as Dy has not been fully clarified yet, it is presumed that the R2T14B-type, main-phase crystal grain particles having a higher concentration of heavy rare earth elements such as Dy than that of the crystal grain boundary phase contributes to achieving high Br, while the R2T14B-type, main-phase crystal grain particles having a lower concentration of heavy rare earth elements such as Dy than that of the crystal grain boundary phase contributes to achieving high iHc close to that obtained by the single method.